Higher Thyroid Levels Tied to Risk for Sudden Cardiac Death

Patients with thyroid levels on the higher end of normal have a significantly greater risk of sudden cardiac death, according to new research published in Circulation.

Layal Chaker, MD, MSc, research fellow in endocrinology and epidemiology at Erasmus University Medical Center, Rotterdam, Netherlands, and colleagues evaluated 10 318 patients in the Rotterdam Study with an average age of 64.7 years who underwent thyroid-stimulating hormone (TSH) or free thyroxine (FT4) measurements. Patients were nearly all Caucasian and more than 57% of the population were women.

The Rotterdam Study is a long-term evaluation of heart disease and other chronic disease in middle-aged and elderly individuals in the Netherlands. Dr Chaker and colleagues included 3 cohorts from the Rotterdam Study in their analysis. Rotterdam Study Cohort 1 (RSI) and Rotterdam Study Cohort 2 (RSII) were composed of patients who were at least 55 years old with baseline data collected from 1990 to 1993 and 2001 to 2001, respectively. Patients in the Rotterdam Study Cohort 3 (RSIII) included those who were not previously invited to participate, were at least 45 years old, and had their baseline data collected from 2006 to 2008. Median follow-up was 9.2 years.

The researchers defined euthyroidism as a value within the reference range (0.85–1.95 ng/ dL) and hypothyroidism was defined as TSH of >4.0 mIU/L and FT4 <0.85 ng/dL.

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Using an age- and sex-adjusted Cox proportional-hazards model, the researchers found 261 sudden cardiac death events (3.1 per 1000 person-years); of these patients, 231 euthyroid patients experienced sudden cardiac death events.

There was a hazard ratio (HR) of 1.87 per 1-ng/dL increase of FT4 for patients with an increased risk of sudden cardiac death due to higher FT4 levels (95% CI, 1.09–2.86) and a greater risk of sudden cardiac death events when analyzing only euthyroid patients (HR, 2.26; 95% CI, 1.30–3.94).

“When restricting the analysis to euthyroid participants with normal FT4 values, the absolute 10-year risks increased from 1% to almost 4% with increasing FT4 values,” the researchers wrote in their study.

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Dr Chaker and colleagues also found patients with other factors, such as high blood pressure or high cholesterol, had an increased risk of sudden cardiac death after controlling for these factors in the analysis.

“We know that a considerable proportion of patients on thyroid hormone replacement therapy are overtreated and so have high blood levels of thyroid hormone,” Dr Chaker stated in a press release. “Our study suggests more caution is warranted in the treatment of thyroid hormone replacement. Replacement therapy is often aimed at the high-normal range which carries a risk of overtreatment.”

While the researchers only analyzed 1 baseline measurement of thyroid function, they hope thyroid hormone level measurement can be used to assess individuals or groups at risk for sudden cardiac death.

“Currently, we do not have a good way to predict sudden cardiac death in the general population,” Dr Chaker said in the release. “Thus identifying additional risk factors is crucial. Our results indicate that thyroid hormone levels may be useful for assessing risk to prevent sudden cardiac death.”